As an example for exemplifying the present invention, it is referred hereinafter to mobile data transmission and data services, which are constantly making progress. With the increasing penetration of such services, a need for increased bandwidth for conveying the data is emerging. With the progressive development of new systems and standards, such increased bandwidth and other features and characteristics is/are realized.
For example, LTE-Advance (LTE-A) is the evolution system for LTE in 3GPP, which is targeted to provide a much higher UE (data) throughput, less delay in end-to-end packet transmission, lower costs per bit, as well as to generally improve the performance and to enhance the scheme for more network deployment scenarios.
Among those deployment scenarios, LTE local area (LA) deployment is a very important and hot studied scenario. Such local area LA deployment normally covers a short range e.g. tens of meters, and access of only a small number of UEs is expected. Typically, such local area scenario is for example used to provide for indoor coverage and/or for coverage of company areas (indoor as well as outdoor with limited size).
As mentioned above, the LA deployment in LTE-Advance is expected to provide the short range coverage e.g. tens of meters, and also the number of users and/or UE's accessing and/or randomly accessing such local area network is expected to be rather low, e.g. less then ten UEs. Those UEs will follow existing procedures as defined under LTE and/or LTE-A for gaining random access to a channel.
In brief, currently, there are totally six events to trigger random access in LTE system. Those events are
1) Initial access from RRC_IDLE
2) RRC Connection Re-establishment procedure
3) Handover
4) DL data arrival and UL non-sync during RRC_CONNECTED requiring random access procedure
5) UL data arrival and UL non-sync during RRC_CONNECTED requiring random access procedure
6) For positioning purpose during RRC_CONNECTED requiring random access procedure
For event 2) mentioned above, there are five initializations, which are:
1) Radio link failure
2) handover failure
3) mobility from E-UTRA failure
4) integrity check failure indication from lower layers
5) RRC connection reconfiguration failure
Since a local area LA has only a small coverage, a radio link between LA UE and LA eNB will probably be good enough to prevent initialization reasons 1) and 4) to occur. But still, at least initialization 2), 3) and 5) could still occur in a LA scenario. Thus, for example, event 2) could trigger RACH in LA scenario.
Among those earlier mentioned six events to trigger a RACH procedure, at least for events 1), 2) and 5) a contention based RACH procedure is required, since eNB does not know when any of the events happens and thus is not able to schedule a non-contention based RACH.
Normally contention based RACH include 4 steps and correspondingly 4 messages:
Step#1: UE sends a preamble to the eNB (Msg1)
Step#2: eNB sends (Msg2) a random access response, RAR, to the requesting UE;
Step#3: UE in turn sends Msg3 to eNB;
Step#4: eNB sends Msg4 to UE; this could be UL grant, DL assignment, UE contention resolution identity etc.
However, there are delays involved in contention based RACH procedures, which are based on the following reasons. From step#1 to step#2, at least 4 ms are required; from step#2 to step#3, at least 6 ms are required; from step#3 to step#4, at least 4 ms are required.
Hence, the minimum time for such RACH procedure takes 14 ms. But to achieve this minimum time, all the following conditions should be fulfilled simultaneously, which assumes that the RAR target for this UE is correctly received at the first subframe of RAR window; no delay configured for Msg3 and Msg3 is correctly received by eNB at the first transmission; Msg4 is correctly received by UE at the first possible chance and contention resolution is successful at the same time; there is no backoff, no Msg3 retransmissions, no preamble retransmission during the whole RACH procedure.
Obviously this is not the normal case for contention based RACH even for LA scenario. And the delay for contention based RACH could thus probably exceed thousands of millisecond for the worst case.
Based on these characteristics of a LA deployment, LTE features and procedure could be further optimized and enhanced to provide higher UE throughput and fewer delay. In particular, the RACH procedure in LA deployment is still desired to be improved in order to reach the target mentioned above.
It is thus an object of the present invention to improve local area deployments such as those under LTE and/or LTE-A.